Effect of chemical groups of polystyrene membrane surface on its pervaporation performance

The relationships between the kinds of chemical groups on membrane surface and pervaporation performance are very important for both the surface design of high-performance membrane and the mechanism study of pervaporation separation. In this paper, polystyrene (PS) membranes surface with carboxyl, hydroxyl and phenyl groups were prepared by casting α,ω-dicarboxyl- (PS-(COOH)2), α,ω-dihydroxyl- (PS-(OH)2) and proton-(PS-H) terminated polystyrene solution on glass plate, respectively. Hydrophilic end groups are localized at the glass-side surface of PS-(COOH)2 and PS-(OH)2 membranes in order to minimize the interfacial free energy between the membrane and the glass plate, which was documented by XPS analysis, surface energy and contact angle measurements. Phenyl groups are localized at the air-side surface of PS-H membrane. This study focuses on the effects of these surface chemical groups on membrane permselectivity for an aqueous solution of ethanol in pervaporation. The kinds of groups on PS membrane surface play an important role in separation factor and the separation factor was in the order as follows: –COOH > –OH> phenyl. However, the chemical groups on the membrane surface did not make any influence in flux. This means that separation factor of polystyrene membrane was influenced by its surface characteristics, however, the flux was determined by bulk properties. The water selectivity of PS membrane increases with increasing the content of hydrophilic groups on membrane surface, which was prepared by adding end-functionalized polystyrene with small molecular weight to PS-H. At the same time, the water concentration in the permeate increased with operating time when the air-side surface faced to feed mixture during pervaporation. The reason was attributed to surface reconstruction that polar carboxyl or hydroxyl groups migrate to the surface creating a “hydrophilic conformation” when the air-side surface of PS-(COOH)2 and PS-(OH)2 membranes contact an aqueous alcohol solution.